This application is a 371 of PCT/FR98/01829 filed, Aug. 20, 1998.
This invention relates to the domain of water treatment.
More precisely, the invention relates to a physical or physicochemical process designed to reduce the content of suspended matter or parasites in this water, and the installation corresponding to this process.
The invention may be used to treat any water containing suspended matter when it is required to reduce the content, particularly such as:
waste water that has already been treated biologically in order to further purify it,
residual water that will be reused after treatment, for example in industry or agriculture, particularly for irrigation;
water to be discharged into the sea;
previously treated waste water (biologically or by other means), in order to control the residual phosphorus content of the discharge.
As described below, the method and plant according to the invention will be particularly useful to treat suspended matter contained in water to be reused for irrigation.
There are several state-of-the-art treatment techniques for suspended matter. These techniques include:
physical sedimentation or preferably physicochemical sedimentation consisting of subjecting the water to clarification, and possibly also adding a coagulating reagent in order to increase the sedimentation capacity of the suspended matter;
filtration on a sand bed, usually between 1.5 and 2 m thick.
Treatment speeds used on conventional sedimentation tanks rarely exceed 2 to 2.5 m/h. These treatment speeds can be increased up to between 10 and 15 m/h when a lamellar type of sedimentation tank is used, and up to 20 m/h when lamellar sedimentation is combined with a coagulation-flocculation step.
In order to obtain good water quality, prior art recommends that sand beds should be used with a maximum filtration speed of 10 m/h.
Therefore, these two treatment types both have the disadvantage that they cannot be used at high speeds, which makes it necessary to use physically large plant involving expensive civil works.
The main objective of this invention is to propose a water treatment method designed to reduce the content of suspended matter and also if applicable of parasites (particularly helminth eggs) so that the structures used can be more compact than state-of-the-art plants, while obtaining a sufficient quality of the treated water to satisfy all requirements, for example for reuse as described above.
This objective is achieved according to the invention which relates to a process for the treatment of waste water in order to reduce its content of suspended matter, and possibly parasites, characterized in that it consists of passing the said water through at least one sedimentation tank at a treatment speed higher than 20 m/h and then upwards through at least one multi-layer filter with decreasing size grading at a treatment speed higher than 10 m/h.
Therefore, the invention proposes to treat the suspended matter and if applicable the parasites, by using a sedimentation tank operating at high speed (in other words a speed higher than the speed conventionally recommended by prior art for the use of this type of sedimentation tank) and a multi-layer filter with size grading decreasing in the upwards direction of the filtration flow, also operating at high speed.
As described above, there are known ways of treatment of suspended matter by sedimentation or by filtration on a sand bed. However, to the knowledge of the applicant, it has never been proposed to:
combine this type of sedimentation tank with a granular filter, particularly with a specific type of filter, namely a multi-layer filter with decreasing size grading and upwards current,
or use high treatment speeds in this type of device.
Very surprisingly, the applicant found that the combined use of treatment speeds higher than speeds recommended in the state of the art both in the sedimentation tank and in the granular filter, could result in:
a very good reduction of suspended matter and parasites such as helminth eggs, and,
significantly lower clogging rates on the filter than are obtained with conventional filtration.
This result is not obvious from prior art which does not recommend the combination of a sedimentation tank and an upwards flowing multi-layer filter, and which requires treatment speeds much lower than those proposed by the invention to obtain a good degree of purification. This result is also contrary to what an expert in the subject would expect, since it is usually accepted that the treatment quality reduces as the treatment speed increases.
According to one preferred variant of the invention, the said process comprises an intermediate step which consists of screening the said water between its outlet from the said sedimentation tank and its inlet into the said filter. The use of this type of screening protects the false bottom of the granular filter, in other words prevents the accumulation of sludge in the space provided in the lower part of the filter under the granular material, into which water is directed from the sedimentation tank. This type of screening can also retain fibers and any other clogging materials that could be contained in the water from the sedimentation tank.
This intermediate step has major advantages compared with screening normally done at the beginning of treatment normally recommended according to the state-of-the-art, it is more economic since it is used on water with a lower content of materials, and can protect the filters more directly even when there is a failure in the sedimentation system.
It would be possible to use the sedimentation tank without the addition of a coagulating agent. In this case, treatment speeds of up to 30 m/h are possible with this system.
However, the sedimentation tank used for the embodiment of the process according to the invention will preferably be a ballasted floc sedimentation tank with at least one coagulating agent. In this case, the treatment speed in the sedimentation tank can be at least 35 m/h. The best choice is a sedimentation tank with lamellar ballasted floc. Under these circumstances, the treatment speed used can be at least 60 m/h and can be up to 200 m/h.
These speeds are much higher than speeds recommended by prior art for this type of installation which, as already mentioned above, does not exceed 20 m/h, even in the case of lamellar ballasted sedimentation tanks.
This type of sedimentation tank is described particularly in the French patents FR 2,627,704 and FR 2,719,734. Therefore, their use within the framework of the process according to the invention can further increase treatment speeds and therefore the compactness of the structures.
Ordinarily, treatment methods involving flocculation-sedimentation are used together with a coagulating reagent injection system designed to quickly and vigorously disperse the reagent in the water to be treated.
Contrary to conventional wisdom in prior art, the applicant recommends that the coagulating reagent should be injected directly into the fast mixing tank in the ballasted floc sedimentation tank in an area where turbulence is low and away from the main currents, or that the reagent should be injected after being very diluted so that the coagulant can be injected in the form of iron or aluminum precipitates composed mostly of hydroxides.
It was found, surprisingly, that the use of this type of injection method could significantly limit or check the elimination of phosphates, without affecting the reduction in suspended matter.
This adaptation of the process is particularly useful in the case of water to be reused for irrigation, such that it maintains a sufficient quantity of phosphates.
In this preferred variant, the possibility of controlling the residual phosphorus as a function of the application type without affecting the reduction in suspended matter, without any limitations on the quantities of added reagents or the very low values of residual phosphorus, is particularly useful.
According to one preferred variant of the method, it comprises an additional step. consisting of injecting at least one flocculating reagent in the said water on the inlet side of the said filter.
According to another preferred variant of the invention, the treatment speed used in the multi-layer granular filter with upwards flow is between 25 m/h and 45 m/h.
According to another aspect of the invention, the filter is washed preferably by passing water output from the sedimentation tank as a backwash through the said filter and then diverting the dirty water current output from the filter. This type of washing can be done without the use of pumps or a filtered wash water storage tank, thus reducing the cost and the complexity of the method.
When it is decided to use several filters in parallel, a filter can be washed using water from the sedimentation tank simply by increasing the water flow through the said filter, and decreasing the flow through one or several other filters.
This type of washing operation can be carried out with concomitant, continuous or intermittent, injection of air into the filter material, in order to improve its efficiency. It would also be possible to systematically, or occasionally, inject a disinfectant such as chlorine in the wash water.
Also according to a variant, the process according to the invention can be used on the inlet side of a biological waste water treatment plant. It can also be used on the outlet side of. a treatment plant, for which it can improve the performances and/or operating conditions.
The invention also relates to an installation for the use of the method described above characterized in that it is composed of the combination of at least one sedimentation tank followed by at least one multi-layer filter with size grading decreasing in the upwards direction of filtration.
Preferably, the installation comprises at least one screen provided between the said sedimentation tank and the said filter with a mesh of between 0.5 and 5 mm, and preferably between 500 xcexcm and 2 mm.
Advantageously, the said sedimentation tank is a sedimentation tank with ballasted floc, and is preferably lamellar.
According to one variant of the invention, the said multi-layer filter comprises at least one gravel bed in at least one sand bed.
Preferably, the installation also comprises a disinfecting device located on the inlet side of the said filter.
Finally, also preferably, the installation comprises means of extracting at least part of the matter retained during the filtration cycle in the bottom of the said filter, by one or several gravity flushes.